Lol I've always dressed more 'modest' than people would 'expect' from someone of my age (I use scare quotes because what is considered 'modest' anyway?) I used to think that this was the main culprit in people thinking that I'm older than I am, but it turns out that it's the whole package (e.g. the way you speak & hold yourself, as others have mentioned) that sways people to consider me as more 'mature' (to the point that one random woman told 14 year old me that I 'keep myself so young' because she thought my sister was my daughter!)
I never used to mind it but my mother would always make a point of it by comparing me to others (e.g. my sister, who is 7 years younger than me - hence the woman's comment above), which would make me very self-conscious about trying out many trends in my teenage years, which is difficult to do when also keeping to a strict modesty code (ironically enforced by my parents).
My answer was therefore to try less covered up outfits when I grew up & moved away. But I always felt a little awkward in them; even without my mother's input, they just didn't feel like 'me'. Which is why I started turning to the internet to try & figure things out beyond simply trends; I had my colours & style diagnosis 'done' last year, which has been eye opening. Whilst I don't look good in the very stark lines of traditional business clothing, I also look too 'try hard' in most 90s grunge looks & outright 'sloppy' in the 70s or 2000s boho styles, but very 'twee' in overly frilly looks. A lot of it has to do with my natural features (that don't change so much over time) - I have a softer face but I'm not a 'lush' type of hourglass like Kibbe's romantic body type (I'm still trying to get my head around those categories but I think I'm more like a Soft Classic). So I'm still feeling my way around my style but I think I've finally started listening to my need for a more covered look (just because I feel more comfortable that way) but also my preference for just a little softness but in a 'womanly' rather than 'girly' way (e.g. looking for pussybow blouses rather than ruffled or frilly ones).
So this is where vintage & thrifting enters! @lyn & @unfrumped mentioned being limited by retail options - I mentioned the eras above because they heavily influenced how awkward I felt getting dressed then because of how difficult it was to find my preferred styles. I often had an idea in my head but could never execute it in reality. I've moved around a lot in tiny living spaces, but I'm hoping that once I settle down I can get a proper sewing machine to just start churning out my own creations. But in the meanwhile, Hong Kong's secondhand charity shops have really stepped up their game, so I'm taking full advantage - my current goal is to get pussybow blouses, boatneck tops & maxi skirts in (almost) every solid colour of the rainbow. So long as I can add in a few trendy accessories here & there, I'm content!
Sorry this is a very long ramble but I suppose what I'm trying to say is that, whilst there may indeed be changes brought about by my body & environment over time, I think that once you figure out the basics of feeling more comfortable in your skin altogether, it can become easier to piece everything together later on. I like Audrey's videos (she had a very useful one on the 'pale olive' skintones, which really helped me to pinpoint why most makeup was making me look too ashy or yellow) so I largely agree with what she's saying here. But that may change as I get older &, as @Joy so eloquently put it, I discover the new ways in which my body may change (I already apply suncream every day though, don't worry But I already prefer a little stomach coverage with everything starting to head south). I'm intrigued by how things may continue to change, though - hopefully for a while yet!